[V134-specific] Slash non-signers upon consensus

[V134-specific]

Intended to close #1454 (closed)
'Late-observation consensus slashes happen with perverse-incentive timing'.

Also closes #1639 (closed) '!2994's implementation of GetConsensusSigners is incomplete'
and its !3068 (closed) 'Use GetConsensusSigners for pre-confirmation-counting observed txin consensus slashing as well'.

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2023-08-13 update:
Rebased, also updating to include something I had previously overlooked (noticed relating to #1639 (closed)),
handler_observed_txin (pre-confirmation-count) non-Final observations.